Fixing embargo when visibility is set via default

I am pretending to set a default. In the request cycle we instantiate
the object, then set the attributes via parameters. If the object sets
defaults on instantiation, then this fix ensures that you can set the
embargo release date via the parameters.

Example:

```ruby
class MyModel
  attribute :visibility, default: 'private'
  attribute :embargo_release_date
end

class MyModelsController
  def create
    @my_model = MyModel.new

    @my_model.attributes = params[:my_model]
  end
end
```

@NOTE - It is unlikely that a default embargo visibility and release
date will properly play together.
Merge pull request #534 from ndlib/fixing-embargo-for-default-being-set

Fixing embargo when visibility is set via default